1. Field of Invention
This invention relates to an apparatus and a method for simultaneous crystal diameter measurement and melt level measurement to be used in an apparatus for growing crystals by the Czocharalski technique.
2. Description of Related Art
The Czocharalski method for crystal pulling is used to provide crystal rods for the semiconductor industry. The Czocharalski method involves melting high-purity semiconductor material in a crucible that is contained within a housing having a non-reactive atmosphere, and maintaining the temperature of the melt at a temperature just above the freezing point. A seed crystal is dipped at a particular orientation into the melt, and is then slowly raised from the melt so that liquid from the melt adheres to the withdrawing crystal seed. As the seed crystal with its adhering material is pulled away from the melt, an elongated crystal rod is formed. The diameter of the crystal rod is a function of many variables. For example, the temperature of the melt in the vicinity of the liquid-solid phase battery, i.e., adjacent to the interface between the melt and the growing rod, affects the rod diameter. The pulling rate and the melt level also affect rod diameter. Variations in rod diameter can result in excessive waste because the rod is then trimmed and cut to produce wafer slices of uniform physical dimensions.
The crystal rod is usually subjected to a grinding process before it is cut into wafers for making, for example, integrated circuit devices. In the grinding process, the outer peripheral surface of the silicon monocrystal rod is ground until a predetermined diameter is obtained. Because of purity concerns, the material ground from the silicon crystal rod cannot be reused. Accordingly, any removed material is considered waste, which results in undesirable increases in production cost. For example, for an unground bar 600 mm in length and 158 mm in diameter, if the diameter is reduced by one millimeter through grinding, the volume of the removed material is equal to 298 cubic centimeters. Accordingly, careful control of the crystal rod diameter is desirable.
A conventional apparatus used for crystal diameter measurement is shown in FIG. 1. In FIG. 1, a luminous ring 74 is formed at an interface between the surface of a silicon melt 42, which is contained in a crucible 24, and the single silicon crystal 40 being pulled out of the melt 42. The luminous ring 74 is imaged to produce a video signal that is sent to an automatic control system. Imaging can be performed using a CCD camera 52, for example. A particular portion of the video signal such as an inner diameter D.sub.i 70 or an outer diameter D.sub.O 72 contained within the image, is detected and multiplied by pre-calculated constants to determine the diameter of the crystal 40 being grown. However, this diameter is calculated with the assumption that the center of the crystal 40 is at a known location. Thus, the automatic control system uses the location of the optically detected luminous ring 74 to calculate the distance from the detected point on the luminous ring 74 to a theoretical center of the crystal 40 in order to calculate a radius, which is then converted to a diameter.
Although this technique of converting points on the luminous ring 74 to compute a diameter is generally effective, there are several factors that can affect the accuracy of the diameter calculations. One key factor involves a phenomena known as orbit. As the crystal 40 grows, it is rotated around its longitudinal axis at a controlled rate. If the mass of the crystal 40 or the various mechanical parts used to hold and raise the crystal 40 from the melt 42 are not perfectly centered along the longitudinal axis of the crystal 40, the entire crystal 40 swings in a circular motion. The swinging motion is known as orbit. As the crystal 40 orbits in the melt 42, the luminous ring 74 moves, causing the automatic control system to incorrectly detect a change in diameter of the crystal 40, rather than sensing the orbit. The automatic control system then tries to compensate for the incorrectly perceived diameter change by adjusting temperature and/or pulling speed, resulting in a loss of diameter control.
Another phenomena affecting the diameter calculations when using the above-described method is the location of the melt level in the crucible 24. Because the CCD camera 52 is mounted above the melt level and uses an oblique angle to monitor the luminous ring 74, the melt level becomes critical. As depicted in FIG. 1, the CCD camera 52 is placed above the melt level at an oblique camera angle .alpha., for example 22.5 degrees. If the melt level is in the theoretically predicted spot, a accurate diameter measurement can be calculated. If the melt level is higher or lower than theoretically calculated, the measured diameter of the crystal 40 is off by the difference between a theoretical and actual melt level multiplied by the tangent of the camera angle .alpha. and multiplied by two since the measured reading is the radius. In general, the error in diameter measurement due to melt level can be calculated as: EQU D.sub.e =L.sub.e .times.tan .alpha..times.2 Equation 1
where,
Le is the error in melt level, measured along the PA1 longitudinal crystal axis, PA1 .alpha. is the camera angle, PA1 D.sub.e is the resulting error in diameter measurement.
The camera angle .alpha. plays another critical role in diameter measurement accuracy. The CCD camera 52 is calibrated to a known angle .alpha. relation to the viewing area. This angle .alpha. is used in the mathematical algorithms for calculating the diameter D of the crystal 40. If the CCD camera 52 is bumped or moved after calibration, the theoretical calculations are incorrect due to the predicted sensing location and the actual sensing location not being identical.
Conventional apparatuses for controlling crystal growth generally attempt to measure crystal diameter but do not compute melt level error. For example, U.S. Pat. No. 4,915,775, which is hereby incorporated by reference, discloses an apparatus for adjusting the initial melt level of a liquid crystal. A vertical position of the melt surface during crystal growing is calculated from a reduction rate of the melt. U.S. Pat. No. 4,350,557, which is hereby incorporated by reference, discloses an apparatus that controls the cross-sectional configuration of the crystal by first rotating the crystal and then detecting the circumference of the crystal. This apparatus does not measure or compute melt level. U.S. Pat. No. 3,998,598, which is hereby incorporated by reference, controls the growth of the crystal by regulating the pull rate of the crystal and the temperature of the melt, and by raising or lowering the crucible in accordance with the pull rate. A pyrometer aligned with the liquid-solid interface of the melt and the crystal provides a signal when the interface is out of alignment. IBM Technical Disclosure Bulletin, Vol. 27, No. 10A, "Monitoring Diameter Variations and Diameter Control Using Laser Beam and Image Processing in Czocharalski Crystal Growth," which is hereby incorporated by reference, uses a reflected laser beam to measure changes in the meniscus and to compute a corresponding diameter change. Finally, Digges, Jr. et al., which is hereby incorporated by reference discloses how optical sensing of the luminous ring can be used to indirectly measure and control the crystal diameter.
Using the conventional systems described above, the total diameter error is the sum of the orbit error, twice the melt level error and the CCD camera angle error. These errors can counteract each other such as having one error indicate a smaller than actual diameter while another indicates a larger than actual diameter. However, all the errors could also indicate a diameter that is either too small or too large. This compounding of errors is a worst case scenario, and can result in significant material loss due to oversized or undersized crystals.